1. Field of the Invention
The present invention relates to a serial dot matrix printer and particularly relates to a serial dot matrix printer for carrying out a printing operation with a printing head mounted thereon while a carriage having the printing head is shuttled between respective ends of a printing medium.
2. Description of the Prior Art
Generally, a dot printer requires that mechanisms be moved at a high speed and positioning be performed accurately. To this end, a method has been employed in which a slit encoder and a photo sensor (hereinafter referred to as a photo encoder in combination) are associated with a carriage moving mechanism so that the stroke of movement of a carriage is controlled/divided and the dot timing is adjusted to suppress the displacement of dots between the forward and backward movements of the carriage. The method is used in a printer of the shuttle type in which a carriage is always moved right and left with a fixed stroke.
That is, in the printer of the shuttle type, the number of dots in a stroke can be made small so that photo encoder functions equal in number to the dots can be provided on a mechanism portion, such as a cam or the like, which is associated with the movement of the carriage and in which the actual distance of movement of the carriage is mechanically enlarged. Further, an inexpensive photo sensor may be used as the photo encoder.
On the other hand, in a serial dot matrix printer (hereinafter referred to a serial printer), the movement of a carriage is controlled on the basis of the number of steps of a stepping motor so that a dot command is applied to a printing head at positions determined by time-dividing the length of each step. For example, if a step has a length of 0.28 mm, the length is divided into four, as a result of which a dot interval is made to be 0.07 mm.
This is because the stroke in the serial printer is several times longer than that in the printer of the shuttle type, and the carriage is operated to return back from a given position.
In that case, as the carriage moving mechanism, it is necessary to use a motor which can be driven by very fast clocks so that the motor can follow the period of dots of the printing head (that is, a period in which outputs can be continuously provided).
If a photo encoder is used also in the serial printer similar to the printer of the shuttle type so that the photo encoder is associated with the carriage per se to thereby control the position of movement of the head actually moved by the carriage, displacement of dots can be suppressed as a result of which the printing speed can be made higher. An experiment performed by the inventor of this application has proved that if the absolute position of the carriage is continuously detected/controlled with a pitch smaller than about 0.7 mm, the printing operation can be performed with regular dot intervals of 0.07 mm at a speed of one dot per msec even if variations are generated in the moving speed of the carriage to an extent.
The continuous detection and control of the absolute position of the carriage with a pitch smaller than about 0.7 mm can be satisfied by use of a photo encoder substantially equal in manufacturing cost as well as in accuracy to that used in the printer of the shuttle type.
However, if such a photo encoder as described above is used as it is in the serial printer, the following problems will be caused.
That is, the serial printer operates to return the carriage from a given position as described above, and therefore the serial printer exhibits a displacement from vibration of 0.3 mm or about 2 mm depending on a machine, is repeated when the carriage stops to perform a return. Accordingly, even if the printer is the best machine (0.3 mm), photo slits formed at intervals smaller than about 0.7 mm may be counted a plural number of times, so that the absolute position of the carriage is inaccurately determined. Therefore, displacement of dots is caused between forward and backward movement of the carriage. Accordingly, the method using a photo encoder can not be employed in print with high quality such as graphic printing or the like, and therefore the method has been used with the printing speed lowered and in unidirectional printing.